This invention relates to shell supported tubular vessels such as grinding mills and more particularly to a journal and end closure for such mills.
Prior to the present invention, the type of mill to which the present invention relates was generally known. This type of mill and the bearing support arrangement is generally described in U.S. Pat. No. 3,093,426 to A. S. Cornford. Other type of bearings for similar mills are shown in U.S. Pat. Nos. 4,025,133 and 3,887,245. With this type of a mill and bearing arrangement, a cast head and trunnion is not used to close the mill end and form the bearing and journal for supporting the mill. The shell itself serves as the bearing journal.
When compared to other types of mill bearing arrangements, a shell mounted bearing arrangement has the advantage of eliminating heavy castings which can present manufacturing problems such as long delivery times and defective castings. Another advantage of shell mounted bearings for large rotating vessels is that the rotary vessel is supported over a larger diameter than with a head and trunnion arrangement. This serves to reduce the bending moments in the shell itself.
With shell mounted bearing arrangements for tubular vessels, the bearing journal is a heavy section in the vessel shell, usually at each end of the vessel. The bearings usually take the form of a hydrodynamic slide shoe mounted on a frame. During rotation of the vessel a high pressure film of oil separates the slide shoe and journal.
The end closure of the mill of this type is formed by an annular plate or vestigial head welded to the inside of the mill at the journal, perpendicular to the shell. An end plate is secured to the vestigial head to form part of the inlet or outlet of the mill. Prior to the present invention, the vestigial head was welded directly to the heavy plate which forms the bearing journal of the shell. The heavy welds required resulted in the formation of lamellar tears in the base metal of the mill shell. These lamellar tears substantially weaken the mill shell and can result in ultimate failure of the mill at the juncture of the mill head and mill shell. This failure can be troublesome particularly if the mill is used in a wet grinding process in that the failure manifests itself as cracks in the various welds. Wet material leaks through these cracks causing housekeeping problems within the milling facility.
As the tubular vessel rotates, cyclical stresses are produced at the juncture of the vestigial head and the vessel shell. Although the stresses per se are not excessively high, their cyclical nature and the fact that a grinding mill must be capable of withstanding many years of operation and thus many cycles, can lead to failure of the welds at the juncture of the vestigial head and journal.